What was to become California State Route 56 was added to the State Highway System by 1959 Legislative Chapter 1062 as Legislative Route Number 278 (LRN 278). LRN 278 as originally adopted was part of the Freeways & Expressways System. The original definition of LRN 278 was as follows:
"LRN 2 (US Route 101/Interstate 5) north of La Jolla to LRN 198 (California State Route 67)"
LRN 278 appears for the first time on the 1960 Division of Highways Map.
As part of the 1964 State Highway Renumbering the Legislative Route Numbers were dropped and reassigned with Sign Route numbers. LRN 278 was subsequently reassigned as California State Route 56. The planned California State Route 56 appears for the first time on the 1964 Division of Highways Map.
The May/June 1965 California Highways & Public Works announced the alignment adoption of the entire planned 17 miles of California State Route 56 between Interstate 5 and California State Route 67. The entire alignment of California State Route 56 was adopted by the California Highway Commision during their March/April Meetings.